Every mechanical structure is inherently subjected to mechanical stress of one type or another. Where the structure is located in a gravitational field, for example, the mere presence of the gravitational force causes certain strain to be placed upon the component parts of that structure. Most physical structures are engineered to withstand such background stress; however, in many environments, it is not unusual for abnormal stresses to be applied to such structures, from time-to-time. These abnormal stresses may come from environmental events, such as earthquakes, but also from the use of the structure, such as a bridge or from operation of the structure, such as a machine, in the normal course of activity.
For example, buildings receive variable loading conditions as a result of human use, wind currents, vibrations, etc. Bridges are subjected to varying loads as a result of traffic thereacross (and the vibrations caused thereby), wind currents and tidal currents, to name a few. Where any structure is coupled to the ground, this structure may be subjected to thermal loads, soil/structure interaction (creep, settlements, and consolidation) and unusual loads caused by earthquakes or other natural disasters. Machinery, such as ships, aircraft, industrial equipment and the like, encounter varying load forces as a condition of their operation since the machinery normally operates by applying a force to some resistance in order to produce work. Thus, the structural members of machinery must bear the loads caused by the application of the force.
The present invention concerns measuring strains on structural systems, although the present invention can be used to monitor strain between any two locations susceptible to the attachment of a strain element according to the present invention. Thus, not only can the present invention monitor strain on structural members, but also the present invention can measure dislocation between two points and can monitor strain, for example, in a soil or substrate medium.
The need for strain measuring devices is unquestioned. It is well known that natural disasters often excessively strain a structure so as to make that structure unfit for occupation or use. Indeed, even a normal use of a structure or normal use of machinery may degrade the system beyond tolerable levels. In either event, current precautionary techniques require visual inspection of the structure, and, sometimes, the inspector must make numerous empirical measurements in an effort to estimate degradation of the structural system. In large constructions, such as buildings, dams, roadways, and the like, it is not only possible but also probable that structural damage due to strain is suffered by the internal structural members of the system. In such instances, damage is almost impossible to estimate due to the inaccessibility of the structural members to observation. Even where the structural members comprising the structural system may be viewed and/or measured, the evaluation process is time consuming and costly. Where only a limited number of inspectors are available, a backlog can result preventing occupancy and use of the structures, for example, as would occur after an earthquake or natural disaster. Further, where a structural member has be strained, but subsequently returns to its original shape or to a less strained state, inspection may not reveal the true magnitude of degradation. Indeed, it is therefore possible that unsafe structures could pass inspection where the strain degradations are hidden.
The existence of strain gauges, as analytical apparatus, has heretofore been known. For example, electrical strain gauges have been developed and may be normally grouped into four types: (1) capacitance gauges: (2) inductance or magnetic gauges; (3) piezoelectric gauges; and (4) resistance gauges. Of these, one of the most prevalent is the inductance gauge wherein a measurement of strain, as a function in a change in length, occurs by the displacement of an inductive element relative to a conductive coil so as to change the inductance of the electronic circuit resulting in a change in current flow therethrough. This change in current flow creates a strain signal that may then be magnified by any suitable electronic means. Similarly, capacitive strain gauges operate by relative movement of capacitive plates or a dielectric medium to change the capacitance of the system thereby generating a signal indicative of relative movement between the points subjected to strain. Resistive strain gauges react to changes in resistance on wires due to strain deformation of the wire. Piezoelectric strain gauges operate on the current generation of piezoelectric crystals placed under strain to produce a strain measurement.
Despite the development of these strain gauges, little has been done to incorporate strain gauges in structural systems as an analytical tool to measure stress on the structure. In part, this may be due to the high cost of existent strain gauges, but also, these gauges are fairly complex in construction and relatively fragile. Thus, operation and measurement requires a relatively high level of skill, and constant use in situ would likely lead to unacceptably high maintenance and lack of reliability. Accordingly, there remains a need for an improved strain gauge element and strain gauge devices which are relatively inexpensive to produce and easy to use and operate.